1. Field of the Invention
The present invention concerns a method for the row-by-row control of a coplanar sustaining AC type of plasma panel, particularly a plasma panel wherein each elementary picture element is defined substantially at the intersection of a first electrode, called a "column electrode", with two other parallel electrodes, called "sustaining electrodes".
2. Description of the Prior Art
Plasma panels are flat panel or screen display devices that enable the display of alphanumerical, graphic or other images, in color or otherwise. These panels work on the principle of an emission of light produced by an electrical discharge in a gas.
Generally, plasma panels comprise two insulating plates bounding a volume occupied by a gas (generally a neon-based mixture). These plates support conductive electrodes intersecting so as to define a matrix of picture elements or pixels. An electrical discharge in the gas, causing an emission of light at a cell or pixel, takes place when the electrodes of this pixel are suitably excited.
Although certain plasma panels work in DC mode, it is most commonly preferred to use AC type panels, the working of which is based on an excitation of the electrodes in AC mode. The electrodes are coated with a layer of dielectric material. They are therefore no longer in direct contact either with the gas or with the discharge.
The working of an AC type plasma panel, with two intersecting electrodes, to define a pixel is known, notably from a French patent No. 78 04893, filed on behalf of THOMSON-CSF and published under No. 2 417 848. This patent also describes a method for the erasure of the pixels of a panel such as this, as well as various types of signals that are applied to the cells (the gaseous space between two intersecting electrodes, i.e. at the pixel) of a plasma panel, notably writing, sustaining and erasing signals:
The writing signal is formed by a voltage pulse, with an amplitude at least equal to the triggering voltage of the gas of the cell. The cell emits a brief light pulse, for the electrical charges created by ionization of the gas cannot reach the electrodes, which are isolated by dielectric layers. These charges get deposited on the dielectric layers and create an internal electrical field that counters the electrical field induced by the writing signal and grows until it causes the cell or pixel to be extinguished. The cell keeps the previously acquired internal field in memory, and it is then said to be at the state 1 or written state, whereas a pixel having an almost null internal field is said to be at the state 0 or erased state. Thus, the writing signal enables the cells ot pixels that are at the state 0 to be set at the state 1.
The sustaining signal stores the information of a cell in the "written" state. Thus sustaining signal is formed by an AC voltage which, twice per period, lights up a cell which is already in the written state. The internal field, memorized by a cell or pixel in the written state, makes it possible to light up this pixel by a sustaining signal with an amplitude that is smaller than the triggering voltage. At each ionization of the gas of the cell or pixel, caused by a sustaining discharge, the internal field gets cancelled and an internal field with a sign opposite to the previous one charges the cell or pixel.
The erasing signal enables one or more or all the cells or pixels of the panel to be placed in the state 0 or erased state. The erasing signal does not modify the state of the cells that are already in the state 0. The erasure of a cell consists in causing a triggering of erasure, namely an ionization of the gas of this cell with, for example, an intensity that is just enough to cancel the charges that have collected on the dielectric layers facing the electrodes. Thus, for example, there are known ways to erase a cell in the state 1, in using a voltage pulse, calibrated in time and amplitude, which ionizes the gas of the cell and cancels its internal field, without generating a new field, unlike what is obtained with a sustaining signal. To this effect, it is possible to use a voltage pulse in the form of square waves, having either a high amplitude and a short duration or a low amplitude and a long duration.
The above-mentioned patent application further explains how the erasure or one or more cells is done by means of an erasing signal, the rising edge of which is formed by a slope.
With a view, notably, to improving the luminance of the plasma panels and also to enabling the display of several colors, it is preferred to use plasma panels which are of the type excited in AC mode a mentioned above and which, in addition, have coplanar sustaining. In panels of this latter type, called coplanar sustaining plasma panels, each pixel of the matrix is formed by three electrodes, more precisely at the intersection between an addressing electrode, called a column electrode, and two parallel sustaining electrodes forming a pair of sustaining electrodes. In this type of panel, the sustaining of the discharges is done between the two sustaining electrodes of one and the same pair, and the addressing is done by the generation of discharges between two intersecting electrodes. The term "addressing" refers to discharges generated selectively or semi-selectively in order to achieve a writing or erasing operation.
Thus the sustaining electrodes form two classes: the electrodes of a first class are called "addressing-sustaining" electrodes, while the electrodes of a second class are called "solely sustaining electrodes". The addressing-sustaining electrodes have the function of setting up the sustaining discharges in cooperation with the solely sustaining electrodes (of the second class). But they also have to fulfil an addressing role. Consequently, they are individualized, that is, they must, for example, bed connected to one or more pulse generating devices through means that enable one or more particular pulses to be applied to only one or to more addressing-sustaining electrodes which are selected from among the plurality of addressing-sustaining electrodes.
Of course, the column electrodes are also individualized.
As for the solely sustaining electrodes (of the second class), they are generally connected to one or more pulse generators in such a way that these solely sustaining electrodes are all, at the same instants, carried to the same potentials, so that they do not need to be individualized and may, if necessary, be connected to one another.
Among the advantages provided by the structures where a pixel is defined at the intersection of a column electrode with a pair of sustaining electrodes, we might cite greater luminance. This is due notably to the fact that the sustaining discharges between the two sustaining electrodes occur on a surface that goes beyond the surface of intersection with the column electrode. This means that the useful light is not blocked by this column electrode which is generally mounted on the side with the plate by which the plasma panel is looked at.
It must be noted that the addressing/sustaining electrodes and solely sustaining electrodes each have, at each pixel, a protuberance or projecting surface. In one and the same pair of sustaining electrodes, the projecting surfaces of one electrode are pointed towards the projecting surfaces of the other electrode, and the sustaining discharges occur between these projecting surfaces.
A plasma panel such as this is known notably from the European patent document EP-A-O 135 382 which also describes a method for the control of this panel. It must be noted that, in the device described in this European patent, the column electrode intersects the pairs of sustaining electrodes on the side of the projecting surfaces where the sustaining discharges are produced.
Another structure of the type wherein each pixel is defined at the intersection of a column electrode with a pair of sustaining electrodes, as well as an adapted control method, are described in the article by G.W. DICK in PROCEEDINGS OF THE SID, vol 27/3, 1986, pages 183-187. It must be noted that, in the structure described in this document, the sustaining electrodes have a constant width, that is, they have no facing, projecting surfaces in a pair of sustaining electrodes, to define the sustaining discharge zone. By contrast, this structure has barriers made of an insulating material. These barriers serve to confine sustaining discharges in the zone of intersection with the column electrode.
Another type of plasma panel, to which the method of the invention can be applied in a particularly worthwhile way, is shown in FIG. 1. A panel of this type is the object, in itself, of a French patent application No. 88 03953 filed on 25th Mar. 1988 on behalf of THOMSON-CSF. Since this French patent application has not been published to date, the new type of plasma panel to which it refers is described hereinafter.
The panel shown in FIG. 1 has a first glass plate 10 covered with a first class of electrodes marked Xj where j is a whole number ranging from 1 to N (only one electrode Xj is shown; the set formed by the plate 10 and the electrode Xj is coated with a layer 12 of dielectric material, which may be covered with a layer of oxide such as MgO (not shown), facilitating electronic emission. On the dielectric layer 12, there is a patch 14 of a luminophor material, namely a material capable of emitting a colored radiation under the effect of an ultra-violet radiation.
The panel further has a second glass plate 20 coated with a second class of electrodes formed by pairs of electrodes, respectively called sustaining-addressing electrodes (Yae)i and sustaining electrodes (Ye) where i is a whole number in the range of 1 to P. The sustaining-addressing and sustaining electrodes include protruberances or projecting surfaces 22 and 24, placed so as to face each other. The set formed by the plate 20 and the electrodes is coated with a dielectric layer 26.
In normal operation, the two plates 10 and 20 and their networks of electrodes are brought close together and kept apart by a shim (not shown), there is a gas in the volume between the plates and the shim. Once the panel is mounted, it thus has two networks of orthogonal electrodes, in the sense that the electrodes Xj are orthogonal to the electrodes (Yae)i and (Ye). The electrodes Xj may overlap the protruberances 22 and 24, or may be slightly offset on their side. A pixel Pij is then defined by an electrode Xj (a column electrode) and a pair of sustaining electrodes (Yae)i and (Ye).
If the above-described plasma panel or the other previously described plasma panes are controlled by a known control method, it is observed, the working of these panels may have one or more of the faults mentioned below:
The pulses applied to the different electrodes may have many levels of voltages, resulting in a complication of the pulse generators and of the number of selective addressing means;
The duration of the total cycle is long, resulting in incompatibility with operation in fast systems of the video type for example (by analogy with the pictures produced by cathode-ray tubes where an image is defined line by line) and possibly resulting in low luminance due to the low frequency of the sustaining discharges;
The writing and/or erasure of the pixels requires several discharges with the column electrode, resulting in requires several discharges with the column electrode, the possible result of this being a highly accelerated degradation of the luminiphors (used in the latest technologies to modify the coloring of the light emitted).